foldscope

[Jeswin]

You guys hear of this? Sounds interesting. They say it can do 2000x
magnification and cost only 50 cents.

http://www.foldscope.com/

[Simon Quellen Field]

Magnification is not a good metric to use.

The web site claims 800 nm resolution, which is reasonable.

But magnified 2,000 times, that 800 nm is 1.6 millimeters, which is a little large for a pixel.

At 300 pixels per inch (such as a cheap printer or a good cell phone), you get a magnification of 105x.

To calculate the numerical aperture of the lens, we take 1.22 times the wavelength of the light we are using (we'll use 500 nm for green) and divide by the resolution. We get 0.7625.

A decent 40x objective lens has a numerical aperture of about 0.65, and a nice 60x objective has a numerical aperture of 0.85, so this lens lies somewhere in-between, so it is a pretty nice lens.

I am assuming that the folding microscope does not have a substage condenser, which would double the resolution, so the lens would only need to be half as good. A compound microscope with a cheap 20x objective (0.40 numerical aperture) and a matching substage condenser would have slightly better resolution (762.5 nm). That microscope would generally use a 10x eyepiece, giving a magnification of 200x. Printed on paper, that would be 166 pixels per inch, which is still quite a bit of "empty magnification".

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[Jeswin]

On Tue, Mar 11, 2014 at 1:04 PM, Simon Quellen Field <sfi...@scitoys.com> wrote:
> Magnification is not a good metric to use.
> The web site claims 800 nm resolution, which is reasonable.
> But magnified 2,000 times, that 800 nm is 1.6 millimeters, which is a little
> large for a pixel.
> At 300 pixels per inch (such as a cheap printer or a good cell phone), you
> get a magnification of 105x.
>

Do you know how to determine the resolution of a microscope? Or is
that not possible without knowing the numerical aperture?

[Simon Quellen Field]

I discus how to calculate numerical aperture here.

Then I discus resolution in terms of NA.

Microscope objectives have the NA clearly marked, and the objective (along with the condenser) is what determines resolution. The eyepiece just magnifies, without improving resolution.

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